This invention relates to bubble (magnetic domain) memories and, in particular, to a method of resistance matching, useful for forming detectors for bubble memory chips, or stated another way, to an on-chip method of balancing bubble memory detectors, and to a balanced detector formed thereby.
Bubble memory chips have means for forming and supporting bubbles under a suitable magnetic bias field and for propagating bubbles on propagate elements, serially, as a stream in response to a rotating in-plane magnetic field are, of course, old at this time.
The use of magnetoresistive detectors, usually formed of chevron type permalloy propagate elements, which change resistance due to the presence of a bubble thereunder are old in the art. These detectors are usually arranged in a bridge network of two or four legs formed by the magnetoresistive detectors and connected to a suitable source of current. In the two legged bridge, one leg forms an active detector and the other leg forms a dummy detector; the former being the detector through which the bubbles are propagated and the latter, while subject to the same in-plane rotating magnetic field, is out of the bubble path and is for the purpose of providing a noise signal approximately the same in amplitude, frequency and phase as the signal representing the absence of a bubble in the active leg of the detector. In the case of a four legged bridge arrangement, there are two active detectors and two dummy detectors while in the case of the two legged bridge network balancing resistors or constant current sources are used; all for the purpose of maintaining the current through each leg of the bridge is the same.
Thus, when a bubble is expanded and propagated through the active detector, a signal of one amplitude is generated due to the change in resistance of the active detector elements, but, on the other hand, if there is no bubble under the active detector, a signal of lesser amplitude is generated and both these signals are sensed by a read-out device, such as a differential amplifier. The differential amplifier often used is a Motorola sense amplifier MC1544/1444 which includes a clamp circuit (capacitor restore), to set the voltage level at which the generated signal is to be compared, and a strobe circuit to sample the signal at a preselected time. This amplifier has a gain which is sensitive to the imbalance occurring across the input terminals at the time the amplifier is unclamped. Thus, if the balance across the bridge network is not a good one, then the signal to the amplifier is unbalanced at all times which reduces the gain of the amplifier to the point where detection becomes marginal.
Typically, to block the DC component signal, i.e., the offset signal, due to the unbalance of the bridge, capacitors and hold-down resistors are introduced between the bridge and the amplifier and accordingly, it is one object of this invention to eliminate these parts.
It should be pointed out also, that with the present trend toward a reduction in magnetic bubble sizes, and concomitantly, a decrease in the cross sectional areas of the magnetoresistive elements themselves, contributes to the offset signal problem due to the unbalance of the bridge network.
This invention shows how a balancing of the bridge network can be achieved without putting impractical constraints on processing techniques and improving the chance of utilizing smaller bubbles, thus increasing bit density and reducing the cost of bubble memories.
Accordingly, it is an object of this invention to provide a method of on-chip balancing of detectors to eliminate the offset voltage problem imposed by the conventional sense amplifier.
A more specific object of this invention is to provide a method of balancing a bridge network detector arrangement for bubble memory chips by providing a balancing resistor ladder of a plurality of resistance elements which can be selectively disconnected.
Another specific object of the invention is to provide a balancing resistor ladder arrangement on a magnetic bubble chip comprising a plurality of resistance which can be selectively disconnected to balance an on-chip bubble detector bridge arrangement.